(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of avoiding a buried contact trench and reducing sheet resistance in the fabrication of integrated circuit devices.
(2) Description of the Prior Art
A typical buried contact is formed by depositing a doped layer of polysilicon over and on the planned buried contact regions and heating the structure. The buried contact regions are doped by outdiffusion of dopants from the doped polysilicon layer into the silicon substrate. The doped polysilicon layer is allowed to remain on the buried contact regions as their contacts. For submicron devices, this diffusion method does not provide adequate control. Alternatively, the buried contact regions may first be formed by ion implantation before the polysilicon layer is deposited thereover. Buried contacts are used for reduction of cell area, for example, in static random access memory (SRAM) devices. In order to reduce sheet resistance between the buried contact and the N+ source/drain, the extension of the polysilicon gate to the buried contact is very small. Hence, it is easy to induce a buried contact trench during etching of the polysilicon if there is even a slight misalignment of the mask. A buried contact trench will increase junction leakage and sheet resistance between the buried contact and the N+ source/drain.
U.S. Pat. No. 5,576,242 to Liu teaches forming a sidewall spacer in the recess of a second polysilicon layer to prevent etching of a buried contact trench. U.S. Pat. No. 5,654,231 to Liang et al teaches forming a spacer on the sidewall of the first polysilicon layer within the buried contact opening to prevent etching a trench. U.S. Pat. No. 5,494,848 to Chin discloses an enlarged photoresist mask over the buried contact area to prevent etching a trench during overetch of the polysilicon. U.S. Pat. No. 5,674,770 to Lee et al describes the split polysilicon method. U.S. Pat. No. 5,292,676 to Manning increases the buried contact implantation area by descumming the photoresist mask.